This invention concerns a method for protection of the variator in a continuously variable belt-type transmission.
A continuously variable automatic transmission usually comprises, among others, a start-up unit, a forward/reverse drive unit, an intermediate shaft, a differential, hydraulic and electronic control devices and a variator. The variator usually includes a primary and a secondary pulley also called primary and secondary side, wherein both pulleys are formed by bevel pulleys arranged in pairs and are provided with a torque transmitting belt-type element which rotates between the two pairs of bevel pulleys. In such a transmission the actual ratio is defined by the running radius of the belt type element which, in turn, is function of the axial position of the bevel pulleys. A continuously variable transmission (CVT transmission) is consequently a positively engaged transmission; the engine torque is transmitted by (static) friction between the belt-type element and the variator pulleys.
According to the prior art, the variator is mostly functionally divided in one side for control of the change of ratio (adjustment) and the other side for setting the necessary contact pressures (tightening) which ensure the required contact between the pulley and the belt-type element so that no slip generates. Depending on the operation setup either the primary or the secondary pulley is used for providing the contact pressures.
For this purpose the transmission control sets, in on the variator a hydraulic contact pressure in order to be able to transmit the momentary (engine) torque. The main input variables for this pressure are the momentary transmission input torque and the transmission ratio. The contact pressure should be as high as needed and as low as possible. If the pressure is too low, skidding (slip) of the variator results and thus damage to the transmission. If the pressure is too high, the transmission efficiency us unnecessarily impaired.
In order to prevent slippage between the pulley and the belt-type element, the prior art teaches the use of additional safety measures or safety factors on the calculated control pressure values. This procedure has the disadvantage that the efficiency of the automatic transmission is thereby impaired. Therefore, the additional safety measures (safety factor) must be selected to be as small as possible.
When the engine torque increases quickly, e.g. because the driver gives more gas, then simultaneously with the actual engine torque at the latest, the necessary contact pressure must be provided in the variator. For this is needed a torque signal that anticipates the actual torque, since the transmission needs a certain amount of time to calculate and provide the necessary pressures.
Such an anticipated torque signal is the driver""s desired torque M_mot_driver""s wish. The signal is also used as input variable for the transmission control, however, in order not to influence the driving properties negatively, the anticipation needs to be very limited in time.
Furthermore, the hydraulic (controlled) system deteriorates due to low temperatures or pollution; this anticipation may not be sufficient and slipping of the variator and thus harm to the transmission can result.
Also, even in a fixed operating condition, an error in the electronic or hydraulic system can lead to underpressure and thus to damage of the transmission when the added safety measure (safety factor) is not sufficient.
The problem on which this invention is based is to indicate, departing from the cited prior art, a method for protection of the variator in continuously variable belt-type transmissions so as to achieve the best possible efficiency of the transmission and, at the same time, optimize the prevention of damage to the transmission.
The inventive method must adapt the contact pressure to the operation point position so that it is possible to reduce the effects of an overpressure to a minimum that is relevant to the efficiency.
Accordingly, it is proposed simultaneously with the calculation based on an engine signal containing the magnitude of the engine torque and subsequent conversion of a contact pressure theoretical value loaded with added safety measures, to measure by a pressure sensor the contact pressure on the variator and to calculate with reference to this value the permissible maximum transmissible transmission input torque. The maximum transmissible transmission input torque is then compared with the engine torque to the transmission and in case the maximum transmissible transmission input torque is less than the engine torque, a controlled engine interruption is effected by the differential amount (engine torque-maximum transmissible transmission input torque), which reduces the engine torque to the transmissible value.
The engine signal is preferably a torque signal anticipating tile actual torque, e.g. the already mentioned driver""s desired torque M_mot_driver""s wish.
It is further proposed for the safety factors, to be stored as characteristic field, additionally to store adaptive correction characteristic fields for added safety of the contact pressure, wherein the relationship of temperature, load and reduction ratio is taken into account so as far as possible to prevent an engine interruption. It thus can be ensured that at operation temperature the safety factors do not unnecessarily increase.
The method can preferably be applied with the aid of a closed loop structure with the theoretical value of the safety factor Sf_soll as a command variable, the actual value of the safety factor Sf as controlled variable and the engine torque M_soll as correcting variable.
The advantages of the inventive method are:
A permanent protection of the variator is ensured by a controlled engine interruption when during critical situations the hydraulic contact pressure is not enough, the necessary safety factors being adapted according to temperature, load and reduction ratio and stored as correction characteristic field.
The inventive method makes possible an optimum efficiency of the transmission by minimizing at an operation temperature, the safety reserves which are not greatly required due to the possibility of an engine interruption.
One other advantage consists in that an online correction in the hydraulic control unit or in the engine control is possible throughout the service life of the system. The correction values can further be used for diagnosis purposes (earlier detection of error, error message, activation of substitute functions).